Pumps for dosing, or injecting, liquids into liquid streams are known. Such devices have been described in, for example, German Published Applications (DOS) Nos. 2,352,677 and 2,600,572, and, while these devices are self-priming, self-ventilating and selfcleaning, they require an additional float chamber associated with the piston dosing pump to protect the device from running dry. Another disadvantage of these known devices is that though water flows into the float chamber and into the pump chamber when the device is started, the pressure of the water in the pump chamber is not as high as that of the water in the liquid conduit and thus priming difficulties can appear in the piston dosing pump.